1. Field of the Invention
The present invention relates to a passive-decoupling receiving antenna for a nuclear magnetic resonance (NMR) imaging apparatus. The invention is more particularly applicable to the medical field in which NMR provides a precious aid in diagnostic techniques. Applications to other fields, however, may also be contemplated.
2. Description of the Prior Art
An apparatus for NMR imaging essentially includes means for subjecting a body to be examined to a constant and intense magnetic field B.sub.O. While being subjected to this influence, the body then receives a radiofrequency excitation from a transmitting antenna in order to induce resonance of oscillation of the magnetic moments of its particles. As soon as this radiofrequency excitation is discontinued, the next step consists in measuring the resonance signal emitted by the body as feedback when the magnetic moments of the particles tend to realign with the field B.sub.O. In order to receive the emitted signal, it is sometimes the practice to employ so-called surface antennas which are placed on the body. The advantage of surface antennas with respect to transmitting antennas forming part of the apparatus essentially lies in the fact that they can be placed in immediate proximity to the emitting particles whereas transmitting antennas are located at a distinctly greater distance. The use of these surface antennas permits a substantially enhanced signal-to-noise ratio of the detected signal.
Surface antennas is most common use (rachis, orbit, breast, knee) are usually spatially decoupled from the transmitting antenna. Said transmitting antenna is usually horizontally polarized so as to minimize eddy-current losses in a patient who is lying in a recumbent position within the apparatus. In the case of surface receiving antennas, these antennas are placed so as to have a vertical polarization of the magnetic field with a view to preventing coupling with the transmitting antenna during transmission. In fact, a strong coupling between the two antennas is highly undesirable since the reactive field induced by the receiving antenna tends to oppose that of the transmitting antenna which serves to excite the protons. At the receiving end, the problem is less crucial since the surface antenna is much closer to the useful signal source.
The spatial decoupling thus proposed is attended by disadvantages in some instances. For example, vertical polarization of the surface antenna may make it necessary for the patient to turn on one side during an examination in which this antenna is applied against his or her ear. Similarly, the patient may also be required to turn over on the couch and lie on the other side if it is desired to place the receiving antenna against his or her other ear.
The aim of the present invention is to overcome these disadvantages by proposing a receiving antenna which is provided with means for decoupling the two antennas during transmission while at the same time ensuring symmetrization of the antenna for receiving the resonance signal as well as converting its impedance for transmission of said signal. In other words, even when placed in a position parallel to a transmitting antenna which is a body antenna, said receiving antenna is insensitive to the uniform field of this latter. This result is obtained without degradation of the signal on reception. This receiving antenna therefore permits observation of any region of the body such as the ear, for instance. In addition, the patient is no longer required to lie on one side.